docs: def result(self, state, action):
          '''Return the resulting state after moving a piece to the empty space.
             (the "action" parameter contains the piece to move)
          '''
          rows = string_to_list(state)
          row_e, col_e = find_location(rows, 'e')
          row_n, col_n = find_location(rows, action)
  
          rows[row_e][col_e], rows[row_n][col_n] = rows[row_n][col_n], rows[row_e][col_e]
  
          return list_to_string(rows)